1. Technical Field
The present invention relates to an AT cut quartz crystal resonator element having a thickness-shear vibration mode as a main vibration and a method for manufacturing the same.
2. Related Art
Generally, a quartz crystal that achieves a high frequency while having stable frequency characteristics is widely adopted as a piezoelectric material for a piezoelectric device such as a resonator, an oscillator, a filter, or a sensor. Especially an AT cut quartz crystal plate is the most widely used material for a quartz crystal resonator having a thickness-shear vibration mode as a main vibration because of the characteristics that a frequency change is small compared with a temperature change around room temperature. The AT cut quartz crystal plate is cut from a quartz crystal, in which a main surface is set by rotating a surface containing an X-axis and a Z-axis around the X-axis counterclockwise from the Z-axis by an angle of 35 degrees 15 minutes.
It has been known that in the piezoelectric resonator element in a thickness-shear vibration mode, where it is formed thinner from a center portion to an end portion, is improved in the frequency characteristics such as CI value or Q value since the amount of vibration attenuation in vibration displacement distribution increases in the end portion to enhance a vibration energy trapping effect in the center portion of the resonator element. JP-A-11-355094 is an example of related art. As a result, the piezoelectric resonator element has the advantage in its capability to resonate effectively with small energy even where the piezoelectric resonator is formed thick to set a frequency low. Conversely, with respect to a comparatively high frequency, this piezoelectric resonator element has the advantage in its capability to come down in size by being set smaller than a regular size. As a shape of the resonator element exhibiting the vibration energy trapping effect, there are a convex shape that a main surface is set to a convex-curved surface, a bevel shape that an interval between a flat thick center portion and an end edge is set to a bevel, a mesa shape that a portion surrounding the flat thick center potion is made thin, and the like.
Such a method has been known to process the piezoelectric resonator element in the convex shape, that a piezoelectric element piece in a strip shape is polished through a mechanical polishing process using a barrel polishing machine or the like. JP-A-2003-205449 and JP-A-8-216014 are examples of related art. Furthermore, the following processing method has also been proposed. Namely, a main surface of the piezoelectric element piece is subjected to a wet etching in a step-by-step manner to process the main surface in a staircase shape which is closely analogous to the convex shape or this staircase shape is reshaped in the convex shape through a machining process using a sandblast, a polishing machine, or the like. JP-A-2003-168941 is an example of related art.
For the bevel shape, the following method has been known. Namely, a quartz crystal piece is processed in a similar manner by mechanical polishing using the barrel machine or is chemically processed by the wet etching using etchant. JP-A-2001-285000 is an example of related art. There is such a piezoelectric resonator that an end portion of the resonator element in the bevel shape is united with a supporting portion in a frame shape to achieve a superior mechanical strength and easy installation. JP-A-11-355094 is an example of related art.
To process the piezoelectric resonator element in the mesa shape, generally, the wet etching is performed to a piezoelectric substrate such as quartz with a patterned electrode film as a mask on a center of a main surface of the substrate, resulting in formation of the thick center portion as a resonating part and the thin surrounding portion. JP-A-2006-140803 is an example of related art. In the mesa-shaped quartz crystal resonator, it has been affirmed that flexural vibration occurring in a longitudinal direction of the substrate is one of causes for spurious response increase. Such a structure has been proposed to reduce this flexural vibration as an unnecessary wave, that a difference in a thickness between the thick center portion and the thin surrounding portion is set between 10% and 30%. JP-A-2006-340023 is an example of related art. Furthermore, such a piezoelectric resonator has been known that an exciting electrode is expanded up to an outside of a step of the thick center portion, in which an elevation changes, so as to achieve prevention of decrease in a capacity ratio, high positional accuracy of the exciting electrode, and prevention of a break in the step. JP-A-2005-94410 is an example of related art.
In the piezoelectric resonator element in the thickness-shear vibration mode, the resonating part is required to be thin in order to set the frequency high. Therefore, such a structure in an inverted mesa shape has been well known, that improves a mechanical strength by uniting a thin resonating part with a surrounding thick reinforcing frame. Furthermore, such a piezoelectric resonator has also been proposed, that is improved in reliability by forming a slit or a flute between the thin resonating part and the reinforcing frame to make it hard to transmit external forces from the reinforcing frame to the resonating part. JP-A-61-189715 and JP-A-10-32456 are examples of related art.
On the other hand, it has been well known that a resonant frequency and frequency temperature characteristics in a thickness-shear mode are highly affected by a high-order width-shear mode having a frequency approximate to that in the thickness-shear mode. Such a piezoelectric resonator has also been proposed in that a union between the thickness-shear vibration and the high-order width-shear mode is weakened by tilting an angle formed between a normal direction of a main surface of the piezoelectric substrate and a side surface in the longitudinal direction by about 3 degrees. JP-A-2001-7677 is an example of related art.
According to JP-A-2001-7677, as shown in FIGS. 7A through 7D, a quartz crystal substrate, in which an X-axis direction of a quartz is set to a long side and a Z′-direction is set to a short side, is processed from an AT cut quartz wafer using a photolithography and an etching process in a manner that an angle, which the normal direction of the main surface of the substrate and each side surface in the longitudinal direction form, is set to about 3 degrees, to be more precise, 3 degrees ±30 minutes in consideration of errors in process. First, as shown in FIG. 7A, both main surfaces of an AT cut quartz crystal substrate 1 having a desired thickness are provided with masks 2a, 2b made from Cr/Au, for example. Next, where the substrate 1 is subjected to the wet etching from both surfaces, an m-face 3 and an r-face 4 as a natural face peculiar to the quartz, and a crystal face 5 that is different from these faces are exposed, as shown in FIG. 7B. The etching is further performed to pass through a part of the quartz crystal where a mask is not formed, so that the m-face and the crystal face 5 become larger because of etching anisotropy of the quartz, resulting in formation of a projection 6 on the side surface of the substrate 1, as shown in FIG. 7C. The etching is yet further performed to become an overetching until when the projection 6 disappears completely, so that such a side surface 7 in the longitudinal direction is formed, that is composed of the crystal face that is tilted at an angle of about 3 degrees with respect to the normal direction of the main surface of the substrate 1, as shown in FIG. 7D.
FIGS. 8A and 8B show the AT cut quartz crystal resonator element using the quartz crystal substrate in a strip shape, which has been processed in the above manner. In the AT cut quartz crystal substrate element 10, a pair of exciting electrodes 12a, 12b is provided onto both main surfaces of a quartz crystal element piece 11 that is formed by eliminating the masks 2a, 2b from the substrate 1 in FIG. 7D. In the quartz crystal element piece 11, the X-direction of the quartz crystal is set to a long side and the Z′-direction is set to a short side. Further, each of the side surfaces 13 in the longitudinal direction is tilted at an angle of about 3 degrees with respect to the normal direction of the main surface. Therefore, the union between the width-shear vibration mode and the thickness-shear vibration mode is set to smaller than that in the case where the tilt angle of the side surface is set to 0 degree, so that the frequency temperature characteristics can be greatly improved.
However, the aforementioned AT cut quartz crystal resonator according to JP-A-2005-94410 has a problem that an energy tapping effect cannot be obtained sufficiently in the Z′-axis direction, i.e., a width direction of the resonator element. FIG. 8C shows vibration displacement in the width direction of the AT cut quartz crystal resonator element 10 shown in FIG. 8B. As is clear from FIG. 8C, the vibration attenuation in the vibration displacement distribution is not enough at both ends in the width direction. Therefore, it becomes difficult to improve the frequency characteristics such as the CI value or the Q value. This has a big effect especially where a downsizing of the quartz crystal resonator element is pursued, since processing accuracy in outline tends to easily vary. Furthermore, a longer etching time than normal is required to perform the overetching to the quartz crystal substrate, leading to possibility of damaging the quartz crystal substrate itself.
As described above, the piezoelectric resonator element in the convex shape achieves the high energy trapping effect but has a problem that it is difficult to process the outline into a desired shape. For example, it is difficult by the mechanical process using the barrel polishing, to control the processing accuracy, resulting in a wide range of variation in a shape and a size. Furthermore, this mechanical process requires the long processing time, leading to decrease in productivity and increase in cost. Yet further, by this process, a surface layer of the quartz crystal, which has been roughened through the polishing process, is needed to be removed by the wet etching before formation of the exciting electrodes. In the barrel polishing, furthermore, since the long sides of the substrate are easily processed, it is difficult to process the substrate in the width direction into the convex shape. The process using the barrel polishing becomes more difficult as the piezoelectric resonator element comes down in size since the element becomes lighter.
The chemical processing method by the wet etching has a problem that the convex shape in an upwardly projecting manner cannot be processed. Especially the process for the staircase shape, which is closely analogous to the convex shape, requires a large number of complicated steps and difficult control, causing a possibility of leading to decrease in productivity and increase in cost instead.
On the other hand, the mesa shape is easily processed by the wet etching utilizing the photolithography and results in a small range of variation in shape, thereby being suitable for mass production. The mesa shape has an advantage in this point over the convex shape and the bevel shape described above but has a problem, due to existence of the step between the thick center portion and the thin surrounding portion, such that the unnecessary wave in the width-shear mode or the like occurs, that superposes on the main vibration in the thickness-shear mode.